There's really no such thing as a family crest.
Perhaps this is a tad pedantic, but the "crest" in a heraldic achievement is just the decoration on top of the helmet. Not all awards of arms have them, and not all heraldic jurisdictions care about them at all.
And then there's the matter that Steven already mentioned in his answer: families don't necessarily have heraldry. Most people -- even nobles -- don't, and even if one person in a family did have a coat of arms, other people in that family -- even the recipient's immediate relatives! -- are not necessarily entitled to bear any heraldry, whether of that same design or any other. It all depends on the specific laws of the specific time and place.